Mechanisms that control normal T-cell homeostasis are not well understood. In this study, we demonstrate
that TSC1 plays a critical role for in maintenance of peripheral T-cell numbers by promoting T-cell survival through the maintenance of normal mitochondrial homeostasis. We have shown that TSC1 inhibits mTORC1 activity, but promotes mTORC2 signaling in T cells. TSC1 may affect mTORC2 signaling through several potential mechanisms. In cell line models, the TSC1/TSC2 complex can associate with mTORC2 to promote mTORC2 signaling 33. In HEK293 cells, it has been demonstrated that Rictor, a component of mTORC2, is directly phosphorylated at Thr1135 by S6K1 after growth learn more factor stimulation, and that this phosphorylation is sensitive to rapamycin. In cells that express a Rictor T1135A mutant, which cannot be phosphorylated by S6K1, mTORC2-dependent Akt phosphorylation was markedly increased, strongly suggesting that mTORC1 activation can directly suppress mTORC2 activity 34. In our model,
TSC1-deficient T cells exhibit highly phosphorylated S6K1 and decreased phosphorylation of Akt and its downstream targets. Whether or not the regulation of mTORC2 by mTORC1, through Rictor, is true during the activation of primary T cells remains to be determined. It has also Selumetinib purchase been reported that elevated S6K1 activity can trigger a negative feedback mechanism to inhibit growth factor induced mTOR activation. For example, S6K1 can phosphorylate IRS-1 to inhibit insulin receptor signaling 35. Elevated S6K1 activity in TSC1 T cells may elicit similar negative feedback inhibition on mTOR-dependent signaling. The exact mechanism as to how TSC1-deficiency leads to mTORC2 inhibition in T cells will require further examination. Our studies indicate that the TSC1/TSC2 complex is paramount for mature T-cell Metalloexopeptidase survival. mTORC2 and Akt activities are decreased in TSC1KO T cells. Since only expression of Akt-DD but not Akt-S473D can rescue
these cells from death, it suggests that the increased death of TSC1KO T cells could not be solely due to decreased mTORC2 activity. The lack of survival defects in Rictor-deficient T cells also supports the idea that mTORC2 is not essential for T-cell survival 10. Increased mTORC1 signaling has been reported to promote cell death. In hepatocyte cell lines, S6K1-deficiency led to down-regulation of caspase-8, caspase-3 activation, cytochrome c release, and protected against the onset of apoptosis 36. S6K1 may promote cell death by inhibiting BAD phosphorylation 37. Since rapamycin cannot rescue TSC1KO T cells from death, enhanced mTORC1 may not directly cause death of these cells. However, this result does not completely rule out a role of increased mTORC1 activity in the death of TSC1KO T cells.